The invention relates to the infusion of dyes into the surface of an article formed of a plastic material using one or more substances each capable of plasticizing the material. More particularly, it relates to treating the surface of an article formed of a plastic material with a solution comprising one or more solvents each aggressive to said material, one or more dyes, and one or more substances capable of plasticizing the material.
Attempts have been made to introduce dyes into articles formed of a plastic material. For optical articles such as lenses, a thermoplastic frequently employed is polycarbonate, which is a condensation product resulting typically from the reaction of bisphenol A derivatives with phosgene. Many dyes degrade at the high molding temperature of polycarbonate. Accordingly it would be advantageous to treat the plastic article after it has been formed.
U.S. Pat. No. 4,657,345 to Gordon utilizes aggressive solvents in combination with various heating steps. However, such surface defects as bubbling, hazing and crazing are caused by retained solvent. U.S. Pat. No. 5,453,100 to Sieloff combines an aggressive solvent with a moderating solvent which acts to dilute and reduce the aggressiveness of the infusing solvent. This approach decreases the volatility of the solvent making it more difficult to drive off, resulting in increased bubble formation at or near the surface of the article. A variant of this approach is to utilize only non-aggressive solvents which reduces solvent penetration and compromises dye density, for example, U.S. Pat. No. 4,043,637 to Hovey, and U.S. Pat. No. 4,323,597 to Olson.
Our invention comprises treating the surface of a previously formed article of plastic material with one or more solvents aggressive to the plastic material having one or more dyes dissolved in the solvent and having further dissolved in the solvent one or more substances capable of plasticizing the material. It is preferable that the material have no plasticizer in it prior to formation of the article, since such would reduce the rigidity, strength and the mechanical properties of the formed article. The aggressive solvent enables penetration to depths in excess of 100 microns to achieve excellent dye density. The plasticizer is thought to provide sufficient polymer mobility to allow the solvent to almost completely escape during subsequent heating without causing bubbling, hazing or crazing. The extent of degradation of mechanical surface properties of the plastic article caused by retained plasticizer is negligible. It is the object of our invention to prevent bubbling and hazing of the surface of an article formed of a plastic material into the surface of which has been infused one or more dyes dissolved in one or more solvents each aggressive to the plastic by also dissolving in the solution one or more substances each capable of plasticizing the plastic material.